ADU Cost on the South Shore | Accessory Dwelling Unit Costs MA | Valcourt Remodeling
Cost Guide · ADUs
ADU Cost on the South Shore
Honest, South Shore-specific cost ranges for accessory dwelling units in 2026, from basement conversions to fully detached cottages. Based on real project history in this market.
Massachusetts ADU law changed in 2025
Chapter 150 of the Acts of 2024 (effective February 2, 2025) allows one ADU by right on every single-family lot statewide. No special permit or variance is required. The state sets a size limit of 900 square feet or 50% of the primary dwelling's floor area, whichever is smaller. Towns can regulate setbacks and design standards, but cannot prohibit ADUs outright. If you were told you could not build an ADU a few years ago, the law has changed.
Price Ranges
Four types of ADU
These ranges reflect full project costs, including design, permits, foundation (where applicable), framing, exterior, interior finish, and mechanical work, for South Shore Massachusetts as of 2026.
Basement Conversion ADU
Converting an existing basement into a self-contained living unit is typically the least expensive ADU path, provided the basement has adequate ceiling height (6'8" minimum per 780 CMR; 8 feet preferred for comfort) and is dry. Costs include egress windows or doors, insulation, framing, electrical, plumbing, HVAC, and full interior finishes. A basement with moisture issues must be waterproofed before finishing, which adds cost.
Garage Conversion ADU
Converting an attached or detached garage into a living unit uses the existing structure as a shell, which reduces cost compared to building from scratch. The work involves insulating walls and ceiling, installing new flooring over the concrete slab, adding HVAC, upgrading electrical, adding plumbing for a bathroom and kitchenette, and modifying the exterior (closing the garage door opening, adding windows and a proper entry door). Detached garage conversions may require a separate utility connection.
Attached ADU Addition
Building a new attached ADU, typically 400 to 800 square feet, involves a new foundation, framing, roofline, exterior finishes, and full interior buildout. This is the most common ADU type we build on the South Shore. An attached ADU shares at least one wall with the main house and typically connects through an interior door that can be locked for privacy. Separate entrance from the exterior is required.
Detached ADU
A fully detached structure on the same lot, such as a backyard cottage, carriage house, or standalone unit, requires its own foundation, complete framing, exterior envelope, and separate utility connections. It offers the most privacy and independence for both households and commands the highest rental rates. On larger South Shore lots in Duxbury, Norwell, Pembroke, and Marshfield, a detached ADU is often the best long-term investment.
Do not budget based on national ADU cost figures
National ADU cost guides often cite $100 to $200 per square foot. Those figures do not reflect South Shore labor rates, the cost of septic upgrades, Conservation Commission requirements, or the complexity of working on older New England homes. The ranges above are based on real project history in this market.
South Shore Factors
What affects ADU cost on the South Shore
These are the variables that most commonly affect ADU project costs in this region, many of which a national cost guide would never mention.
Ceiling height (basement conversions)
The Massachusetts State Building Code (780 CMR) requires a minimum ceiling height of 6'8" in habitable basement spaces. If your basement is right at that threshold, finishing is straightforward. Below 6'4", you may need to lower the floor slab, which adds $15,000–$40,000 depending on the extent of the work and soil conditions.
Separate utility connections (detached ADUs)
A detached ADU typically requires its own electrical service, water connection, and sewer or septic capacity. Depending on the distance from the street and the condition of existing systems, utility connections can add $15,000–$50,000 to a detached ADU project. We assess utility feasibility before finalizing any detached ADU budget.
Septic capacity
Many South Shore properties are on private septic systems. Adding a second dwelling unit increases the load on the system. Before building an ADU, the existing septic system must be evaluated for capacity. Upgrading or replacing a septic system adds $20,000–$60,000 and requires Title 5 inspection and Board of Health approval.
Conservation Commission review
ADU projects within 100 feet of a wetland, stream, or coastal resource area require a Notice of Intent filed with the local Conservation Commission. This adds 4 to 8 weeks to the permitting timeline and requires an engineering consultant. Coastal towns including Hull, Cohasset, and Scituate have significant wetland buffers that affect many properties.
Historic District Commission review
Exterior ADU work in Hingham and Cohasset historic districts requires HDC approval before a building permit is issued. This adds 4 to 6 weeks and requires material selections appropriate to the period of the home. Detached ADUs in these districts must be architecturally compatible with the main house.
Finish level
ADU costs can vary by $30,000–$60,000 on the same footprint depending on finish selections: flooring, cabinetry, tile, fixtures, and appliances. A rental-grade ADU and a high-end in-law suite have very different finish budgets. We help homeowners understand the cost implications of finish choices before they are locked in.
Return on Investment
Why an ADU is one of the best investments on the South Shore
An ADU is not just a construction project. It is a financial asset that generates income, adds property value, and provides flexibility for decades.
Rental income potential
A well-located ADU in Scituate, Hingham, or Duxbury can generate $1,800 to $2,800 per month in rental income. At $2,000 per month, a $200,000 ADU pays for itself in under 9 years and continues generating income indefinitely. Rental demand on the South Shore is strong, particularly for well-finished one-bedroom units near commuter rail.
Property value increase
ADUs add significant value to South Shore properties. A detached ADU in Hingham or Scituate can add $150,000 to $250,000 to your home's appraised value, depending on size, finish, and location. Buyers increasingly seek properties with ADU potential or an existing unit.
Multigenerational living
For families with aging parents or adult children, an ADU provides a solution that is both practical and financially smart. The cost of assisted living in Massachusetts averages $6,000 to $10,000 per month. An ADU that enables a parent to age in place near family is often the most financially sound decision a family can make.
Our Process
How Valcourt Remodeling approaches ADU projects
Free site assessment and feasibility review
Before any drawings are started, we walk the property, review your town's zoning bylaws, assess existing conditions (basement height, septic capacity, utility locations, wetland buffers), and give you an honest picture of what is feasible and what the realistic budget range is. This assessment is included in our pre-construction process at no charge.
Fixed-price contracts
We use fixed-price contracts. The price in your contract is the price you pay, barring unforeseen conditions discovered during demolition or excavation, which we document and discuss with you before proceeding.
OFCI for fixtures and appliances
All plumbing fixtures (faucets, showerheads, tub fillers, toilets), lighting fixtures, and kitchen appliances are owner-furnished, contractor-installed (OFCI). You select and purchase every fixture and appliance to match your exact taste; we handle all installation, rough-in, and connections. Your contract reflects labor and installation costs for these items; the equipment cost is yours to manage directly with your supplier.
Permit coordination
We handle all building, plumbing, and electrical permit applications. For projects requiring Conservation Commission or HDC review, we work with your engineering and design consultants to prepare and submit the required filings. For properties on private septic, we coordinate the Title 5 inspection and Board of Health process.
Architectural integration
An ADU should look like it belongs on your property. We match siding materials, rooflines, window styles, and proportions to the existing home. This is especially important for detached ADUs and attached additions, where a poorly integrated structure can hurt property value rather than help it.
FAQ
ADU Cost South Shore | Common Questions
Find out what your ADU will cost
We offer free, no-obligation site consultations across all nine South Shore towns. We will walk your property, review your goals, and give you an honest picture of what your ADU project will involve.